One manner of controlling steam temperature with varying load conditions of a steam generator is to recirculate gases from the rear gas pass of a furnace back to the furnace. In order to maintain superheat temperature constant regardless of what load the unit is operating at, the recirculation of gases back to the furnace is increased as the load increases. One typical gas recirculation arrangement in use today introduces gas through the furnace wall at a point some distance from the furnace bottom. This has the following disadvantages. Since furnaces are generally lined with welded walls formed of welded together steam generating tubes, the openings are expensive to form. Also, the lower the introduction of the gases into the furnace, the more effective it is in preventing heat transfer to the furnace walls.
Another arrangement in use today, which overcomes the above problems, introduces the recirculated gas through the ash discharge opening in the furnace bottom. This arrangement is typified by U.S. Pat. No. 3,580,226 which issued on May 25, 1971. As mentioned, this patent overcomes the above problems. However, this patent makes use of a liquid valve in the gas recirculation line. The use of the liquid valve places a tortuous path in this line, thus increasing the pressure drop, and thus also the horsepower requirements considerably. This could increase the cost of the unit by more than $100,000.
Another arrangement in use today which overcomes the liquid valve problem is shown in U.S. Pat. No. 2,985,152, which issued on May 23, 1961, where the recirculated gases are introduced into the ash hopper located below the furnace. This arrangement has the disadvantage of increasing the overall height of the unit. Since large steam generators are supported from the top by structural steel, it is desirable to keep the height of a unit to a minimum. An arrangement such as shown in U.S. Pat. No. 2,985,152 could increase the overall height by 4 feet or more, resulting in increased costs of $50,000 to $100,000.